Welding device

ABSTRACT

In a method and device for performing welding, after the welding current reaches a first current value during a short circuit, the feeding of the welding wire is stopped, slowed down, or moved backward. As a result, the opening of a short circuit is urged to reduce the frequency at which welding is unnecessarily interrupted by an overcurrent protection function. This achieves high weld quality such as the absence of welding defects, and high production efficiency.

This application is a divisional of U.S. patent application Ser. No.13/318,648, filed Nov. 3, 2011, which is a U.S. National PhaseApplication of PCT International Application PCT/JP2010/006583, filedNov. 10, 2010, and claims priority to Japanese Patent Application JP2009-267123, filed Nov. 25, 2009, the contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a device for performing welding byfeeding a welding wire and alternately repeating short circuits andarcs.

BACKGROUND ART

In a conventional device for performing welding by automatically feedinga welding wire and alternately repeating short circuits and arcs, ashort circuit is urged to open by supplying a large current to increasethe short-circuit current with time until the short circuit opens and anarc occurs.

In the case, however, that the welding wire is made of a material noteasily meltable due to its low intrinsic resistivity such as aluminum,the welding wire continues to be semi-molten even when a large currentis supplied to open a short circuit. In this case, the short circuitdoes not open, thus failing to create an arc.

If the short circuit does not smoothly open, welding may be interruptedeither due to the buckling of the welding wire or due to the stop of thewelding device in response to an overcurrent protection function. Theovercurrent protection function protects the welding device and weldingjigs from overcurrent.

One well-known method for preventing wire buckling and hence avoidingwelding from being interrupted is to monitor the motor current of thewire feeding motor and to stop the feeding of the welding wire (see, forexample, Patent Literature 1).

The above-mentioned overcurrent protection function has beenconventionally used to protect a welding device and welding jigs at anemergency, such as a short circuit between the tip of the welding wireand the welding output side.

FIG. 7 shows a schematic configuration of a conventional welding device.FIG. 8 shows temporal changes of a welding current and an AS signalwhich indicates an arc state and a short-circuit state in theconventional welding device. The conventionally used overcurrentprotection function is described as follows with reference to FIGS. 7and 8. FIG. 7 shows the overall structure of the conventional weldingdevice having the overcurrent protection function, and FIG. 8 shows therelation between the waveform of a welding output and various timings inthe conventional welding device.

The operation of the welding device shown in FIG. 7 is described withreference to FIG. 8. In the following description, the welding device isa consumable electrode arc welding device which performs welding byalternately repeating short circuits and arcs.

In welding device 1 shown in FIG. 7, welding output unit 2 provides awelding output. Output controller 3 controls welding output unit 2.Current detector 4 detects a welding current. Voltage detector 5 detectsa welding voltage. Feeding motor 6 feeds welding wire 9. Feed roller 8feeds welding wire 9. Base material 12 is an object to be welded, andwelding torch 10 creates arc 11 between welding wire 9 and base material12. Wire feed controller 13 controls feeding motor 6. AS determinationunit 14 determines based on the output of voltage detector 5 whether itis an arc state where welding wire 9 and base material 12 are arced, ora short-circuit state where welding wire 9 and base material 12 areshort circuited.

In FIG. 7, welding output unit 2 receives a commercial electric powerfrom outside welding device 1, and outputs a welding voltage and awelding current by inverter operation in accordance with weldingconditions.

Current detector 4, which can be formed of a current transformer (CT),detects a welding current. Voltage detector 5, which measures thevoltage across the output terminals of welding device 1, detects awelding voltage.

AS determination unit 14, which may be composed of a CPU, receives avoltage detection signal from voltage detector 5. When the weldingvoltage reaches a predetermined detection level (for example, 15V) in ashort circuit state, AS determination unit 14 determines that the ASsignal indicates an arc state (high level). When, on the other hand, thewelding voltage reaches a predetermined detection level (for example,10V) in an arc state, AS determination unit 14 determines that the ASsignal indicates a short-circuit state (low level).

Wire feed controller 13 controls the rotation of feeding motor 6.Feeding motor 6 is connected to feed roller 8 whose rotation allowswelding wire 9 to be pressure-fed at a basic feed speed according towelding conditions.

Welding torch 10 supplies the output of welding output unit 2 to weldingwire 9, thereby creating arc 11 for welding between the tip of weldingwire 9 and base material 12.

In FIG. 8, time point E1 is when a short circuit occurred. Time point E2is when an arc occurred after time point E1. Time point E3 is when ashort circuit occurred after time point E2. Time point E7 is when thecurrent of the short circuit that had occurred at time point E3 reacheda predetermined overcurrent protection detection current value IOC. Timepoint E8 is when an overcurrent protection detection period TOC haspassed since time point E7, and the overcurrent protection functioncomes into operation.

As shown in FIG. 8, since time point E3 at which the short circuitoccurred, the short-circuit current is controlled to increase with anappropriate gradient in order to open the short circuit. Opening a shortcircuit requires outputting an extremely high welding current. Theshort-circuit current is clipped at a maximum output current value IMAX(for example, 550 A) as an upper limit which is determined according tothe performance of welding device 1. The short-circuit current continuesto have the maximum output current value IMAX until the short circuitopens and an arc occurs.

Assume that the welding current reached the overcurrent protectiondetection current value IOC (for example, 500 A) at time point E7, andthat the overcurrent protection detection period TOC (for example, 300msec) passed at time point E8. Also assume that the short circuit didnot open between time points E7 and E8, and that the welding currentcontinued to be equal to or more than the overcurrent protectiondetection current value IOC. In this case, welding device 1 forcefullyterminates its output to protect itself and welding jigs fromovercurrent.

Even when the short circuit cannot be opened by the time the overcurrentprotection detection period TOC passes, welding wire 9 continues to befed to base material 12. The feeding of welding wire 9 continues untilthe overcurrent protection function comes into operation to stop theoutput of welding device 1 and the feeding of welding wire 9. Thiscauses welding wire 9 to be pushed deeply into the weld pool of basematerial 12, making it much more difficult to open the short circuit.

Failing to open the short circuit during the predetermined period causesthe overcurrent protection function to come into operation, andforcefully terminates welding device 1. The forceful termination ofwelding device 1 interrupts the welding. This often damages basematerial 12, causes quality issues such as welding defects, or reducesproduction efficiency.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Unexamined Publication No.2008-68283

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and device forperforming welding as follows. A short circuit that does not easily openis urged to open so as to reduce the frequency at which welding isunnecessarily interrupted by an overcurrent protection function. Thiscan prevent welding defects and improve production efficiency.

To solve the conventional problems, in the method of the presentinvention for performing welding by feeding a welding wire andalternately repeating short circuits and arcs, after the welding currentreaches a predetermined first current value during a short circuit, thefeeding of the welding wire is stopped, slowed down, or moved backward.

This method urges the opening of a short circuit so as to reduce thefrequency at which welding is unnecessarily interrupted by theovercurrent protection function. This achieves high weld quality such asthe absence of welding defects, and high production efficiency such asshort weld time.

A device of the present invention for performing welding by feeding awelding wire and alternately repeating short circuits and arcs includesa feeding motor for feeding the welding wire; a current detector fordetecting a welding current; and a voltage detector for detecting awelding voltage. The device of the present invention further includes anAS determination unit for detecting a short-circuit state and an arcstate based on the output of the voltage detector; an output controllerfor controlling a welding output based on the outputs of the currentdetector, the voltage detector, and the AS determination unit; a weldingoutput unit for generating a welding output based on the output of theoutput controller; a current setting unit for setting a first currentvalue as a threshold for the welding current; and a wire feed controllerfor controlling the feeding of the welding wire based on the outputs ofthe current setting unit, the AS determination unit, and the currentdetector. The feeding of the welding wire is stopped, slowed down, ormoved backward after the welding current reaches the first current valueduring a short circuit.

This structure urges the opening of a short circuit so as to reduce thefrequency at which welding is unnecessarily interrupted by anovercurrent protection function. This achieves high weld quality such asthe absence of welding defects, and high production efficiency such asshort weld time.

A device of the present invention for performing welding by feeding awelding wire and alternately repeating short circuits and arcs, includesa feeding motor for feeding the welding wire; a current detector fordetecting a welding current; a voltage detector for detecting a weldingvoltage; an AS determination unit for detecting a short-circuit stateand an arc state based on the output of the voltage detector; and anoutput controller for controlling a welding output based on the outputsof the current detector, the voltage detector, and the AS determinationunit. The device of the present invention further includes a weldingoutput unit for generating a welding output based on the output of theoutput controller; a current setting unit for setting a first currentvalue as a threshold for a welding current; a time keeper for counting atime since the welding current reached the first current value; anelapsed-time setting unit for setting a first predetermined time, whichis an elapsed-time threshold since the welding current reached the firstcurrent value; and a wire feed controller for controlling the feeding ofthe welding wire based on the output of the current setting unit, thetime keeper, the elapsed-time setting unit, the AS determination unit,and the current detector. The feeding of the welding wire is stopped,slowed down, or moved backward after the first predetermined time haspassed since the welding current reached the first current value.

This structure urges the opening of a short circuit so as to reduce thefrequency at which welding is unnecessarily interrupted by theovercurrent protection function. This achieves high weld quality such asthe absence of welding defects, and high production efficiency such asshort weld time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic configuration of a welding device according toa first exemplary embodiment of the present invention.

FIG. 2 shows temporal changes of a welding current, an AS signal, and awire feed status signal in the welding device according to the firstexemplary embodiment of the present invention.

FIG. 3 shows other temporal changes of the welding current, the ASsignal, and the wire feed status signal in the welding device accordingto the first exemplary embodiment of the present invention.

FIG. 4 is a schematic configuration of a welding device according to asecond exemplary embodiment of the present invention.

FIG. 5 shows temporal changes of a welding current, an AS signal, and awire feed status signal in the welding device according to the secondexemplary embodiment of the present invention.

FIG. 6 shows other temporal changes of the welding current, the ASsignal, and the wire feed status signal in the welding device accordingto the second exemplary embodiment of the present invention.

FIG. 7 is a schematic configuration of a conventional welding device.

FIG. 8 shows temporal changes of a welding current and an AS signalwhich indicates an arc state and a short-circuit state in theconventional welding device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the present invention will be described as follows withreference to drawings. In these drawings, the same reference numeralsare used for the same components, and hence the description thereof maybe omitted.

First Exemplary Embodiment

FIG. 1 shows a schematic configuration of welding device 21 according toa first exemplary embodiment of the present invention. FIGS. 2 and 3show temporal changes of a welding current, an AS signal which indicatesan arc state and a short-circuit state, and a wire feed status signal inthe welding device.

As an example of a welding device, a consumable electrode arc weldingdevice is described as follows, which performs welding by alternatelyrepeating short circuits and arcs. The operation of welding device 21shown in FIG. 1 is described with reference to FIGS. 2 and 3.

In welding device 21 shown in FIG. 1, welding output unit 22 provides awelding output. Output controller 23 controls welding output unit 22.Current detector 24 detects a welding current. Voltage detector 25detects a welding voltage. Feeding motor 26 feeds welding wire 29.Current setting unit 27 sets a threshold for the welding current. Feedroller 28 feeds, via welding torch 30, welding wire 29 to base material32, which is an object to be welded. Arc 31 is created between weldingwire 29 and base material 32. Wire feed controller 33 controls thefeeding motor. AS determination unit 34 determines based on the outputof voltage detector 25 whether it is an arc state where welding wire 29and base material 32 are arced, or a short-circuit state where weldingwire 29 and base material 32 are short circuited.

In FIG. 1, welding output unit 22 of welding device 21 receives acommercial electric power (for example, three-phase 200V) from outsidewelding device 21, and outputs a welding voltage and a-welding currentby inverter operation in accordance with welding conditions.

Welding output unit 22 has an inverter, which can be generally aninsulated gate bipolar transistor (IGBT) or a metal-oxide semiconductorfield effect transistor (MOSFET), which is driven by the pulse widthmodulation (PWM) operation or the phase shift operation.

Current detector 24, which can be formed of a current transformer (CT),detects a welding current. Voltage detector 25, which measures thevoltage across the output terminals of welding device 21, detects awelding voltage.

AS determination unit 34, which may be composed of a CPU, receives avoltage detection signal from voltage detector 25. When the weldingvoltage increased during a short circuit reaches a predetermineddetection level (for example, 15V), AS determination unit 34 determinesthat the AS signal indicates an arc state (hereinafter, “arcdetermination” corresponding to “high level”). When the welding voltagedecreased during an arc reaches a predetermined detection level (forexample, 10V), AS determination unit 34 determines that the AS signalindicates a short-circuit state (hereinafter, “short-circuitdetermination” corresponding to “low level”).

Output controller 23, which may be composed of a CPU, controls weldingoutput unit 22 based on the outputs of current detector 24, voltagedetector 25, and AS determination unit 34. Current setting unit 27,which may also be composed of a CPU, sets a first current value I1 as athreshold for the welding current.

Wire feed controller 33 controls the rotation of feeding motor 26 basedon the outputs of current setting unit 27, AS determination unit 34, andcurrent detector 24. Feeding motor 26 is connected to feed roller 28whose rotation allows welding wire 29 to be pressure-fed at a basic feedspeed according to welding conditions. Welding torch 30 supplies thewelding current and the welding voltage from welding output unit 22 towelding wire 29, thereby creating arc 31 for welding between the tip ofwelding wire 29 and base material 32.

The following is a description, with reference to FIG. 2, of how theopening of a short circuit is urged if the short circuit does not easilyopen.

In FIG. 2, time point E1 is when a short circuit occurred. Time point E2is when an arc occurred after time point E1. Time point E3 is when ashort circuit occurred after time point E2. Time point E4 is when thecurrent of the short circuit that had occurred at time point E3 reachedthe first current value I1, which is a predetermined threshold for thewelding current. Time point E6 is when an arc occurred after time pointE4. IMAX indicates a maximum output current value of welding device 21.

As shown in FIG. 2, the AS signal outputted by AS determination unit 34becomes low (short circuit determination) at time point E1 at which theshort circuit occurred, and becomes high (arc determination) at timepoint E2 at which the arc occurred. When the welding process is stable,the short circuit duration is within 10 msec.

Since time point E3 at which the short circuit occurred, theshort-circuit current is increased with an appropriate gradient in orderto open the short circuit. Opening a short circuit requires supplying anextremely high welding current to welding wire 29. The increasingshort-circuit current has an upper limit of a maximum output currentvalue IMAX (for example, 550 A) which is determined according to theperformance of welding device 21. After reaching the maximum outputcurrent value IMAX, the welding current continues to have the maximumoutput current value IMAX until the short circuit opens and an arcoccurs.

The maximum output current value IMAX is generally determined byabsolute maximum ratings of a semiconductor device such as an IGBT or aMOSFET used in the inverter of welding device 21. Wire feed controller33 sets the wire feed status signal to the ON state (high level) at timepoint E4 after the welding current reached the first current value I1(for example, 400 A) set by current setting unit 27 during the shortcircuit that had occurred at time point E3.

When the wire feed status signal is in the OFF state (low level),welding wire 29 is fed toward base material 32 at the predeterminedbasic feed speed. When the wire feed status signal is in the ON state,on the other hand, wire feed controller 33 stops the operation offeeding motor 26, thereby stopping the feeding of welding wire 29.

When the wire feed status signal is in the ON state, wire feedcontroller 33 may control the operation of feeding motor 26 instead ofstopping it such that the wire feed speed is lower than the basic feedspeed. The basic feed speed is used when the wire feed status signal isin the OFF state as described above. More specifically, the wire feedspeed may be slowed down to 25% of the basic feed speed, a minimumpossible wire feed speed, or the initial wire feed speed at the start ofwelding (generally called a “slow-down speed” which is lower than thespeed in steady-state welding).

Alternatively, when the wire feed status signal is in the ON state, wirefeed controller 33 may control the operation of feeding motor 26 insteadof stopping it such that welding wire 29 is fed backward. Morespecifically, the backward feed speed may be 25% of the basic feedspeed, a minimum possible wire feed speed, or the initial wire feedspeed at the start of welding (generally called a “slow-down speed”).Alternatively, the wire feed speed may be zero (in the stopped state) toavoid feeding welding wire 29 too much when a predetermined time (forexample, 200 msec) has passed since the start of the backward feeding.

Although not illustrated, feed roller 28 has a feed-roller pressuredevice to stop the feeding of welding wire 29. The feed-roller pressuredevice may be a magnetic valve capable of controlling the pressure offeed roller 28. When the wire feed status signal is in the ON state, thefeed-roller pressure device reduces the pressure that is applied by feedroller 28 to welding wire 29. As a result, feed roller 28 is put into anidle state, thereby stopping the feeding of welding wire 29.

Although not illustrated, there is provided a pressure clamp brake suchas a magnetic valve to stop the feeding of welding wire 29. The pressureclamp brake is disposed somewhere along the feed passage of the weldingwire formed, for example, of a conduit cable extending from feed roller28 to welding torch 30. When the wire feed status signal is in the ONstate, the pressure clamping force of the pressure clamp brake mayprovide a frictional resistance to welding wire 29 so as to clamp it,thereby stopping the feeding of welding wire 29.

As shown in FIG. 2, wire feed controller 33 sets the wire feed statussignal to the OFF state (low level) at time point E6 at which the shortcircuit opened and an arc occurred, while the wire feed status signal isin the ON state, and the feeding of welding wire 29 is stopped, sloweddown, or moved backward. Wire feed controller 33 then controls feedingmotor 26 to return the wire feed speed to the original speed (basic feedspeed) that is used before the wire feed status signal is turned on.Thus, the feeding of welding wire 29 is continued.

The overcurrent protection function of the present first exemplaryembodiment is described with reference to FIG. 3. As described above,the maximum output current value IMAX may continue for a predeterminedperiod even when the feeding of welding wire 29 is stopped, slowed down,or moved backward while the wire feed status signal is in the ON statein order to urge the opening of the short circuit. In such a case, theovercurrent protection function operates in the same manner as describedin the BACKGROUND ART with FIG. 8.

In FIG. 3, time point E7 is when the short-circuit current reached apredetermined overcurrent protection detection current value IOC. Timepoint E8 is when a predetermined overcurrent protection detection periodTOC has passed since time point E7 and the overcurrent protectionfunction operates to stop the welding output of welding device 21.

In FIG. 3, since time point E3 at which the short circuit occurred, theshort-circuit current is increased and clamped at a maximum outputcurrent value IMAX (for example, 550 A) as an upper limit. Then, theovercurrent protection detection current value IOC is set to be equal toor lower (for example, 500 A) than the maximum output current valueIMAX. The overcurrent protection detection current value IOC is areference current value used to determine whether the welding outputshould be stopped to protect welding device 21 and welding jigs fromovercurrent. In the example shown in FIG. 3, the overcurrent protectiondetection current value IOC is smaller than the maximum output currentvalue.

In FIG. 3, welding device 21 is forcefully stopped for overcurrentprotection if the short circuit does not open by time point E8 at whichthe overcurrent protection detection time TOC (for example, 300 msec)has passed since time point E7 at which the short-circuit currentexceeded the overcurrent protection detection current value IOC.

As described above, the feeding of welding wire 29 can be stopped,slowed down, or moved backward when the welding current reaches thefirst current value I1 to prevent welding wire 29 from being pusheddeeply into the weld pool of base material 32 during a short circuit.This urges the opening of a short circuit so as to reduce the frequencyat which welding is unnecessarily interrupted by the overcurrentprotection function. As a result, welding defects are prevented, andhigh production efficiency is maintained.

The first current value I1 shown in FIGS. 2 and 3 can be equal to theovercurrent protection current value IOC. In this case, the weldingcurrent can be reliably determined to be short-circuited for a longtime. This prevents feeding motor 26 from being stopped unnecessarilydue to erroneous determination. In addition, this reduces theconsumption of mechanism elements such as the reduction gear of feedingmotor 26 and feed roller 28. As a result, a reliable welding system canbe provided. In the example shown in FIG. 3, the first current value I1is smaller than the overcurrent protection current value IOC.

In the method of the present invention for performing welding by feedingwelding wire 29 and alternately repeating short circuits and arcs, afterthe welding current reaches the predetermined first current value I1during a short circuit, the feeding of welding wire 29 is stopped,slowed down, or moved backward.

This method urges the opening of a short circuit so as to reduce thefrequency at which welding is unnecessarily interrupted by theovercurrent protection function. This achieves high weld quality such asthe absence of welding defects, and high production efficiency such asshort weld time.

In this method, the first current value I1 is equal to the predeterminedovercurrent protection detection current value IOC, which is thereference current value used to determine whether the welding outputshould be stopped to protect welding device 21 from overcurrent.

In this method, the welding current can be reliably determined to beshort-circuited for a long time. This prevents feeding motor 26 frombeing stopped unnecessarily due to erroneous determination. In addition,this reduces the consumption of mechanism elements such as the reductiongear of feeding motor 26 and feed roller 28. As a result, a reliablewelding system can be provided.

Welding device 21 of the present invention for performing welding byfeeding welding wire 29 and alternately repeating short circuits andarcs includes feeding motor 26, current detector 24, voltage detector25, AS determination unit 34, output controller 23, welding output unit22, current setting unit 27, and wire feed controller 33. Feeding motor26 feeds welding wire 29, and current detector 24 detects a weldingcurrent. Voltage detector 25 detects a welding voltage. AS determinationunit 34 detects a short-circuit state or an arc state based on theoutput of voltage detector 25. Output controller 23 controls the weldingoutput based on the outputs of current detector 24, voltage detector 25,and AS determination unit 34. Welding output unit 22 generates a weldingoutput based on the output of output controller 23. Current setting unit27 sets the first current value I1 as the threshold for the weldingcurrent. Wire feed controller 33 controls the feeding of welding wire 29based on the outputs of current setting unit 27, AS determination unit34, and current detector 24. Thus, in welding device 21 of the presentinvention, the feeding of welding wire 29 is stopped, slowed down, ormoved backward when the welding current reaches the first current valueI1 during a short circuit.

This structure urges the opening of a short circuit so as to reduce thefrequency at which welding is unnecessarily interrupted by theovercurrent protection function. This achieves high weld quality such asthe absence of welding defects, and high production efficiency such asshort weld time.

The present first exemplary embodiment has described consumableelectrode short-circuit welding; however, consumable electrode pulsewelding can alternatively be used to obtain similar effects inshort-circuit conditions.

Second Exemplary Embodiment

Welding device 41 and its operation according to a second exemplaryembodiment of the present invention is described with reference to FIGS.4 to 6. FIG. 4 is a schematic configuration of welding device 41. FIGS.5 and 6 show temporal changes of a welding current, an AS signal whichindicates an arc state and a short-circuit state, and a wire feed statussignal in the welding device according to the second exemplaryembodiment.

The main difference of welding device 41 of FIG. 4 from welding device21 of FIG. 1 is to include elapsed-time setting unit 35 and time keeper36, which will be described later.

As shown in FIG. 4, elapsed-time setting unit 35, which may be composedof a CPU, sets a first predetermined time T1 as an elapsed-timethreshold since the welding current reached the first current value I1.Time keeper 36, which may also be composed of a CPU, counts the timesince the welding current reached the first current value I1.

Wire feed controller 33 controls the rotation of feeding motor 26 basedon the outputs of current setting unit 27, time keeper 36, elapsed-timesetting unit 35, AS determination unit 34, and current detector 24.

Feeding motor 26 is connected to feed roller 28 whose rotation allowswelding wire 29 to be pressure-fed at a basic feed speed according towelding conditions. Welding torch 30 supplies the welding current andthe welding voltage from welding output unit 22 to welding wire 29,thereby creating arc 31 for welding between the tip of welding wire 29and base material 32.

The following is a description, with reference to FIG. 5, of how theopening of a short circuit is urged if the short circuit does not easilyopen.

In FIG. 5, time point E5 is when the first predetermined time T1 set byelapsed-time setting unit 35 has passed since time point E4.

In FIG. 5, since time point E3 at which the short circuit occurred, theshort-circuit current is increased with an appropriate gradient in orderto open the short circuit. Opening a short circuit requires supplying anextremely high welding current to welding wire 29. The increasingshort-circuit current has an upper limit of a maximum output currentvalue IMAX (for example, 550 A) which is determined according to theperformance of welding device 41. After reaching the maximum outputcurrent value IMAX, the welding current continues to have the maximumoutput current value IMAX until the short circuit opens and an arcoccurs.

Wire feed controller 33 sets the wire feed status signal to the ON state(high level) at time point E5 during the short circuit that had occurredat time point E3. As described above, time point E5 is when the firstpredetermined time T1 counted by time keeper 36 has passed since timepoint E4 at which the welding current reached the first current value I1(for example, 400 A) set by current setting unit 27. When the wire feedstatus signal is in the ON state, wire feed controller 33 stops theoperation of feeding motor 26, thereby stopping the feeding of weldingwire 29. Alternatively, as in the first exemplary embodiment, thefeeding of welding wire 29 may be slowed down or moved backward insteadof being stopped.

Wire feed controller 33 sets the wire feed status signal to the OFFstate (low level) at time point E6 at which the short circuit opened andan arc occurred. Then, wire feed controller 33 controls feeding motor 26to return the wire feed speed to the original speed (hereinafter, “basicfeed speed”) that is used before the wire feed status signal is turnedon.

The overcurrent protection function of the present exemplary embodimentis described with reference to FIG. 6. As described above, the maximumoutput current value IMAX may continue for a predetermined period evenwhen the feeding of welding wire 29 is stopped, slowed down, or movedbackward while the wire feed status signal is in the ON state in orderto urge the opening of the short circuit. In such a case, theovercurrent protection function operates in the same manner as describedin the BACKGROUND ART with FIG. 8.

In FIG. 6, since time point E3 at which the short circuit occurred, theshort-circuit current is increased and clamped at a maximum outputcurrent value IMAX (for example, 550 A) as an upper limit. Then, theovercurrent protection detection current value IOC is set to be equal toor lower (for example, 500 A) than the maximum output current value. Theovercurrent protection detection current value IOC is a referencecurrent value used to determine whether the welding output should bestopped to protect welding device 41 and welding jigs from overcurrent.In the example shown in FIG. 6, the overcurrent protection detectioncurrent value IOC is smaller than the maximum output current value.

In FIG. 6, welding device 41 is forcefully stopped for overcurrentprotection if the short circuit does not open by time point E8 at whichthe overcurrent protection detection time TOC (for example, 300 msec)has passed since time point E7 at which the short-circuit currentexceeded the overcurrent protection detection current value IOC.

The first predetermined time T1 shown in FIG. 5 may be set shorter, forexample, by 100 msec, than the overcurrent protection detection timeTOC. More specifically, the first predetermined time T1 can be 200 msecor so.

The feed passage through which welding wire 29 is fed is larger thandesigned, and it takes a certain time for feeding motor 26 and feedroller 28 to come to a stop. Therefore, it takes a certain time untilthe tip of welding wire 29 fed from time point E5 in FIG. 5 is stopped,starts to be slowed down, or starts to be moved backward. It generallytakes 100 msec or so, and consequently, the first predetermined time T1can be set shorter by at least 100 msec than the overcurrent protectiondetection time TOC. As a result, the opening of a short circuit can besuccessfully urged.

As described above, in the method of the present invention forperforming welding, the feeding of welding wire 29 is stopped, sloweddown, or moved backward after the first predetermined time T1 has passedsince the time point at which the welding current reached the firstcurrent value I1.

This method prevents welding wire 29 from being pushed deeply into theweld pool of base material 32 during a short circuit, thereby urging theopening of a short circuit. This reduces the frequency at which weldingis unnecessarily interrupted by the overcurrent protection function. Asa result, welding defects are prevented, and high production efficiencyis maintained.

The feeding of welding wire 29 is stopped, slowed down, or movedbackward after the first predetermined time T1 has passed since the timepoint at which the welding current reached the first current value I1.

This method prevents feeding motor 26 from being stopped unnecessarily,and reduces the consumption of mechanism elements such as the reductiongear of feeding motor 26 and feed roller 28. As a result, a reliablewelding system can be provided.

In this method, the first predetermined time T1 is shorter than thepredetermined overcurrent protection detection time TOC, which is thereference time used to determine whether the welding output should bestopped to protect welding device 41 from overcurrent. Morespecifically, the first predetermined time T1 can be shorter by 100 msecor so than the overcurrent protection detection time TOC to ensure tostop welding wire 29 before the overcurrent protection function isoperated. As a result, the opening of a short circuit is successfullyurged.

In this method, when a short circuit opens and an arc occurs after thefeeding of the welding wire is stopped, slowed down, or moved backward,the feeding of the welding wire is returned to the state it was beforebeing stopped, slowed down, or moved backward. This method allows asmooth welding operation.

Welding device 41 of the present invention for performing welding byfeeding welding wire 29 and alternately repeating short circuits andarcs includes feeding motor 26, current detector 24, voltage detector25, AS determination unit 34, output controller 23, welding output unit22, current setting unit 27, and wire feed controller 33. Feeding motor26 feeds a welding wire, and current detector 24 detects a weldingcurrent. Voltage detector 25 detects a welding voltage. AS determinationunit 34 detects a short-circuit state and an arc state based on theoutput of voltage detector 25. Output controller 23 controls the weldingoutput based on the outputs of current detector 24, voltage detector 25,and AS determination unit 34. Welding output unit 22 generates a weldingoutput based on the output of output controller 23. Current setting unit27 sets the first current value I1 as the threshold for the weldingcurrent. Time keeper 36 counts the time since the welding currentreached the first current value I1. Elapsed-time setting unit 35 setsthe first predetermined time T1 as the elapsed-time threshold since thewelding current reached the first current value I1. Wire feed controller33 controls the feeding of welding wire 29 based on the outputs ofcurrent setting unit 27, time keeper 36, elapsed-time setting unit 35,AS determination unit 34, and current detector 24. Welding device 41 ofthe present invention stops, slows down, or moves backward the feedingof the welding wire when the first predetermined time T1 has passedsince the welding current reached the first current value I1.

This structure urges the opening of the short circuit so as to reducethe frequency at which welding is unnecessarily interrupted by theovercurrent protection function. This achieves high weld quality such asthe absence of welding defects, and high production efficiency such asshort weld time.

The present exemplary embodiment has described consumable electrodeshort-circuit welding; however, consumable electrode pulse welding canalternatively be used to obtain similar effects in short-circuitconditions.

INDUSTRIAL APPLICABILITY

As described hereinbefore, the present invention prevents weldingdefects and improves production efficiency by urging the opening of ashort circuit. Therefore, the method and device for performing weldingof the present invention is applicable to fields of constructing LNGtanks, ships, bridges, and other structures where consumable electrodearc welding is used to ensure high weldability of thick boards.

REFERENCE MARKS IN THE DRAWINGS

21, 41 welding device

22 welding output unit

23 output controller

24 current detector

25 voltage detector

26 feeding motor

27 current setting unit

28 feed roller

29 welding wire

30 welding torch

31 arc

32 base material

33 wire feed controller

34 AS determination unit

35 elapsed-time setting unit

36 time keeper.

1. A device for performing welding by feeding a welding wire andalternately repeating short circuits and arcs, the device comprising: afeeding motor for feeding the welding wire; a current detector fordetecting a welding current; a voltage detector for detecting a weldingvoltage; an AS determination unit for detecting a short-circuit stateand an arc state based on an output of the voltage detector; an outputcontroller for controlling a welding output based on an output of thecurrent detector, the output of the voltage detector, and an output ofthe AS determination unit; a welding output unit for generating awelding output based on an output of the output controller; a currentsetting unit for setting a first current value as a threshold for thewelding current; and a wire feed controller for controlling the feedingof the welding wire based on an output of the current setting unit, theoutput of the AS determination unit, and the output of the currentdetector, wherein the feeding of the welding wire is stopped, sloweddown, or moved backward after the welding current reaches the firstcurrent value during a short circuit, the first current value beinglarger than the welding current at a start of the short circuit.
 2. Adevice for performing welding by feeding a welding wire and alternatelyrepeating short circuits and arcs, the device comprising: a feedingmotor for feeding the welding wire; a current detector for detecting awelding current; a voltage detector for detecting a welding voltage; anAS determination unit for detecting a short-circuit state and an arcstate based on an output of the voltage detector; an output controllerfor controlling a welding output based on an output of the currentdetector, the output of the voltage detector, and an output of the ASdetermination unit; a welding output unit for generating a weldingoutput based on an output of the output controller; a current settingunit for setting a first current value as a threshold for the weldingcurrent; a time keeper for counting a time since the welding currentreached the first current value; an elapsed-time setting unit forsetting a first predetermined time, the first predetermined time beingan elapsed-time threshold since the welding current reached the firstcurrent value; and a wire feed controller for controlling the feeding ofthe welding wire based on an output of the current setting unit, anoutput of the time keeper, an output of the elapsed-time setting unit,the output of the AS determination unit, and the output of the currentdetector, wherein the feeding of the welding wire is stopped, sloweddown, or moved backward after the first predetermined time has passedsince the welding current reached the first current value during a shortcircuit, the first current value being larger than the welding currentat a start of the short circuit.
 3. The device of claim 1, wherein thefirst current value is equal to a predetermined overcurrent protectiondetection current value, the predetermined overcurrent protectiondetection current value being a reference current value used todetermine whether a welding output should be stopped in order to protectthe device from overcurrent.
 4. The device of claim 2, wherein thefeeding of the welding wire is stopped, slowed down, or moved backwardafter a first predetermined time has passed since the welding currentreached the first current value; and the first predetermined time isshorter than a predetermined overcurrent protection detection time, thepredetermined overcurrent protection detection time being a referencetime used to determine whether a welding output should be stopped inorder to protect the device from overcurrent.